windows machines need re-building every 6 months to maintain performance, Macs on the other hand don't!
My 1.8 Ghz duo Intel Mac Mini is 5 years old, (been 2 years since a re-build) and its still faster than the HP core2 2.6ghz duo 3gb ram HP piece of crap work give me last month that runs Windows...
I used to be Windows fan, but since got my Mac, never looked back! 
Tunnie - you cant compare a corporate build PC with a home use Mac

. Also, I'm guessing you're not running AV - the killer to Windows if you use the big 2 corporate favourites.
Windows will tend to outperform Mac if hardware is similar due to more efficient kernel and in particular a more efficient display subsystem.
Downsides of Windows is the open architecture, both hardware and software. Every tom dick and harry make shite for it (which is ultimately what kills it), Apple maintain stricter controls...
XP needs regular rebuilds. Vista based kernels seem more robust. Vista does need a bit of RAM, but kernel is better optimised in W7.